Planar cell polarity and the cytoskeleton Planar cell polarity (PCP) signaling regulates the establishment of polarity within the plane of an epithelium. The results of signaling are as diverse as the determination of cell fates, the generation of asymmetric, but highly aligned structures (e.g. stereocilia in the human inner ear or hairs on a fly wing), or the directional migration of cells during convergent extension during vertebrate gastrulation. PCP is governed by the non-canonical Fz/Planar Cell Polarity pathway, in which a Wnt signals through a Frizzled receptor leading to nuclear responses, as well as to cytoskeletal changes mediated by Rho Kinase. PCP signaling was originally discovered and is best studied in Drosophila, mainly because of the versatility of the fly as model system due to its low genetic redundancy. In Drosophila, PCP is essential for the orientation of the actin wing hairs and the polarization of the ommatidia in the eye, requiring highly coordinated movement of groups of photoreceptor cells (ommatidial rotation). Thus, key to PCP signaling in flies, and to convergent extension in vertebrates, are cytoskeletal rearrangements and cell migration processes. Central to these processes is Rho Kinase (Rock, Drok in Drosophila), mutations in which or dominant negative forms of which lead to ommatidial rotation/migration and convergent extension/neural tube defects in flies and fish, respectively. This proposal focuses on biochemical and genetic approaches to identify and characterize new PCP components and regulators of the cytoskeleton. In a genome wide screen we identified direct Drok substrates using a phosphorylation induced gel-shift assay. We will characterize the candidates using in vivo RNAi, mutational analysis and genetic interaction assays with known pathway components. We propose to study their mechanism of action with respect to Rho Kinase and PCP signaling in detail. Using the above strategies to work out mechanistic aspects of PCP signaling as well as to discover new links to cellular responses will extend our knowledge of early development. Due to the conservation of the PCP gene network in organisms as diverse as flies, ascidians and mammals, the proposed research will be of immediate importance for the understanding of the establishment of PCP and related morphogenetic processes in vertebrates.